This type of bearing is particularly efficacious in gas turbine engines where the bearing bore diameter in millimeters times the shaft rotational speed in revolutions per minute (DN) is above two million for supporting the shaft and transmitting radial loads. As gas turbine shaft bearing speeds increase particularly when operating at DN levels in the 2.5 to 3.0 M range, the rotating inner race is subjected to an increasingly unfavorable stress condition that has resulted in rapid race fracture. The acting stress field results from tensile forces produced by a combination of centrifugal effects, thermal effects and the shaft interference fit combined with alternating stresses arising from cyclic roller passing loads. The inner races of these bearings are constructed of a high hardness material (AISI M-50) that has high rolling contact fatigue resistance but which tends to be notch sensitive under tensile type loadings. This notch sensitivity is aggravated by the existence of axial bore slots and intersecting radial holes which are used to supply cooling and lubricating oil to the bearing interior to minimize wear at the cage land and the end surfaces of the roller elements and to maintain bearing thermal stability.
We have found that we can improve the roller bearing having the three piece constructed inner ring type described in the copending patent application cross referenced supra, by fabricating the rolling contact load carrying center section (inner race) from a high strength steel, say M-50 that has no slots or holes and the other two sections which serve to guide the rollers, from a more ductile steel alloy, say AMS6322. By virtue of eliminating the holes and/or slots from the inner race so as not to upset the cross sectional area, the tension and bending strengths are not adversely affected. Further, the two guiding shoulder elements are not similarly subjected to contact fatigue since they only come into contact with the roller ends where the loads are relatively low, thus negating the reason for employing a material that is harder to work and more expensive.
We have found that the bearing life and durability can be improved by judiciously locating the lubricating and oil passages solely in the more ductile and less notch sensitive roller guiding shoulder rings. OIl capturing scoops cooperating with these passages located at either side of the bearing assembly improve the cooling system by enhancing its axial symmetry. It is contemplated that the scoops be made integral or unitary with the roller guiding shoulder rings or can be included with the other proximity elements that stack up the bearing. Conventional practice is to introduce underrace cooling oil assymetrically; that is from one side only. This approach results in thermally induced distortions of a nature that tends to make the inner race assume a conical shape. This in turn produces a maldistribution of stresses that can cause premature failure and wear out. This new concept eliminates this thermal distortion and will accordingly enhance bearing life.
We have also found that bearing durability and life is enhanced by hard facing of both the roller guiding surface and the outside diameter cage land guiding surface of each shoulder ring to provide adequate wear resistance of the relatively soft ductile steel alloy.